This study describes quantitatively the components of atherosclerotic plaques in saphenous vein grafts used for aortocoronary bypass and compares the findings with the plaques in the native coronary arteries in the same men. A total of 607 five-mm segments of saphenous veins and 797 five-mm segments of native coronary arteries were examined by computerized planimetric technique in 19 men, aged 39 to 82 years (mean 61), who had survived bypass operation for more than 1 year. Comparison of the mean percentages of the plaque components in saphenous vein grafts in place for 14 to 26 months with those of the native coronary arteries revealed significant differences: cellular fibrous tissue, 86% vs 7%; dense fibrous tissue, 13% vs 82%; p = less than 0.05. As survival time after the bypass operation increased, composition of the plaques in the saphenous veins changed such that by approximately 80 months the amounts of cellular and dense fibrous tissue in both saphenous vein grafts and native coronary arterie were similar: 10% vs 16%, and 75% vs 71%; ns. Thus, by about 7 years after a coronary bypass operation the composition of plaques in saphenous vein grafts is similar to that in the native coronary arteries of the same patients.